The invention relates to apparatus for separating mixtures of substances, such as solutions, suspensions, emulsions or dispersions, by means of membrane filtration, for example, reverse osmosis, ultrafiltration or dialysis.
In contradistinction to apparatus which operates with mechanical filters, for example, filter presses and rotating settling filters, devices for membrane filtration are suitable also for the separation of lower-molecular and ionic portions. They serve for the selective separation or concentration of individual components of solutions, suspensions, emulsions or dispersions, for example, for concentrating salt water and brackish water, for treating blood, whey, dyestuff suspensions, plastics dispersions, oil/water emulsions or enzymes, for removing acid from wine or for reducing alcohol in beer.
Numerous devices for membrane filtration, also called modules, have been proposed and they may be divided into dynamic and static devices according to their method of operation.
In membrane filtration carried out with dynamic device as described, for example, in German Offenlegungsschrift No. 2,146,867, a higher concentration is usually achieved than with a static device. By rotation of the membrane support and flow interrupter of the dynamic module to produce turbulence, a turbulent flow of the medium to be treated is produced which counteracts too high a concentration and the deposition of solid particles on the surface of the membranes. In comparision with static devices, dynamic devices are expensive technical constructions having relatively high operating costs, and are therefore used only for special purposes.
Among the previously proposed static devices for carrying out membrane filtration are (1) tube modules and hollow fiber modules, which are similar in design to pipe assembly heat exchangers, in which the solution flows in or round circular-cylindrical surfaces, as well as (2) filter press and spiral modules in which the solution flows between parallel membrane surfaces which are either planar or only slightly curved in proportion to the distance between them.
Tube modules, as described, for example, in German Offenlegungsschrift No. 1,805,590, have tubular semi-permeable membranes each attached to a porous supporting tube. Spiral modules, as described, for example, in German Offenlegungsschrift No. 1,442,420, usually consist of flat membranes with liquid-permeable material arranged between each flat membrane being wound spirally around a tube and glued thereto. Filter press modules, which are of similar design to mechanical filter presses, consist, for example, according to German Offenlegungsschrift No. 2,304,702, of a pile of alternating guide plates and membrane support members, each support member supporting a semi-permeable membrane on each side.
Spiral and hollow fiber modules are not suitable for carrying out ultrafiltration, and in the case of reverse osmosis, too, the possibility of using them is limited by the narrow cross-sections of flow for the solution. Particularly in the case of media which on concentration tend to form silt-like precipitates, blockages can occur as a result of the deposition of solids. Hollow fiber modules have the further disadvantage that the operating pressure must remain relatively low.
Filter press and tube modules, however, are suitable both for reverse osmosis and for ultrafiltration, and on account of their large cross-section of flow do not have the above disadvantages of the spiral and hollow fiber modules.
One of the most important requirements in the design of a module is the provision of a packing density of the module which is as large as possible, that is, as large as possible a membrane surface should be arranged in as small as possible a module volume so that with a small plant space requirement an optimal exploitation of construction material is achieved. Filter press and tube modules do not do justice to this requirement; in comparison with other types of modules, they have a relatively low packing density.
Furthermore, previously proposed filter press modules have the disadvantage that replacement of the membrane cannot easily be carried out and there is the danger that the newly introduced membrane will be damaged. Although in U.S. Pat. No. 3,508,994 a filter press module is proposed in which porous carrier members are coated directly with a membrane-forming solution, so that when the membrane is changed it is not necessary to handle the sensitive membrane itself, this module has the disadvantage of having a relatively low packing density.
There is accordingly a need for apparatus suitable for carrying out membrane filtration which has a high module packing density, which is easy to assemble, and which allows the membranes and the members which support them to be changed in a simple manner, and which furthermore operates with as low as possible a local excess concentration and deposition of solids, so that the flux, that is the quantity which permeates per unit of time and per unit of membrane surfaces, remains approximately constant.